Conceptual optical design for CARAMUEL payload: a quantum key distribution system from a GEO satellite
A. Álvarez‐Herrero, Tomás Belenguer, Luis Pascual, Ángel Alvaro, Carlos Miravet, Pablo Campo, Albert Tomàs, Luis Fernando Rodríguez-Ramos, Marcos Reyes, Eloy Sánchez de Gregorio, Félix Cuervo, Pedro Pintó
A feasibility study and the mission definition for a Quantum Key Distribution from a geostationary satellite have been successfully completed by a Spanish consortium led by HISPASAT under ARTES ESA funding. The system will be a hosted payload of a regular communication satellite mission. In this work, the conceptual optical design of the payload is presented. It includes the trade-off between an on-axis telescope with central obscuration and an off-axis unobscured telescope. Besides, the optical design of the transmission (i.e.: quantum signal and downlink beacon) and reception channels (i.e.: uplink beacon) are described and the critical subsystems identified. A polarization control system based on liquid-crystals, which avoids mechanisms, is used to correct the errors due to instrumental residual polarization, reference system rotation or long-term instability of the QKD source during operation.
A feasibility study and the mission definition for a Quantum Key Distribution from a geostationary satellite have been successfully completed by a Spanish consortium led by HISPASAT under ARTES ESA funding. The system will be a hosted payload of a regular communication satellite mission. In this work, the conceptual optical design of the payload is presented. It includes the trade-off between an on-axis telescope with central obscuration and an off-axis unobscured telescope. Besides, the optical design of the transmission (i.e.: quantum signal and downlink beacon) and reception channels (i.e.: uplink beacon) are described and the critical subsystems identified. A polarization control system based on liquid-crystals, which avoids mechanisms, is used to correct the errors due to instrumental residual polarization, reference system rotation or long-term instability of the QKD source during operation.
The usage of QKD protocols through the current terrestrial infrastructure is limited to a hundred kilometers due to attenuation problems with fiber. This implies that the satellite will be necessary for the transmission of quantum keys in order to avoid these terrestrial limitations. Therefore, the current debate does not lie in the need for the satellite but rather in what type of infrastructure will be best suited, and specifically the usage of GEO vs LEO satellites. This analysis is not trivial and should not consider only the link budgets and the physical layer as it has been done up to date. It should also consider even more important factors, as the service KPIs that the end user will request such as key rate, availability, security of the keys and their maintenance, etc., as well as the possible communications architectures to provide the service. Therefore, the analysis should introduce the market oriented component, and provide the optimal solutions based on the scenarios and requirements requested by the customer. In this frame, and under ESA ARTES funding, a Spanish consortium led by Hispasat is defining an operational GEO QKD mission that shall fly as hosted payload in 2025. The quantum communications payload proposed in this mission represents a step beyond the state of the art since it is the first implementation of a quantum key distribution channel from geostationary orbit. This new service will involve the implementation of a complete communications system not currently available that will require the development of new technological elements and the adaptation for their operation in space of technologies such as photon sources, telescopes for optical communications compatible with quantum state transmission or processors for the distillation of quantum keys, as well as a whole new ground segment to control and manage the new quantum key distribution service and its integration into a future European secure quantum communications network. Details of the challenges and approach of this mission shall be presented
Satellite-based Quantum Key Distribution (QKD) can be used to deliver information-theoretically secure keys for cryptographic applications, no matter where end-users are located. Several space missions using this concept have shown feasibility of systems and component technologies. The QUBE-II mission, with a planned launch in Spring 2026, uses an economic CubeSat platform to show the possibility of a global secure communication by simple means. This paper presents the optical system design of the QUBE-II laser communication terminal (LCT). It is based on the previous developments for the CubeL mission (launched in 2021), for CubeISL, and for the precursor mission QUBE-I (launched in 2024). The 2.5U LCT is integrated into an 8U CubeSat and serves as transmit system for two Alice modules operating at 850 nm and in optical L-band1, 2 respectively. At the same time, bi-directional classical links are used for link acquisition, tracking, and data transmission. With its 85 mm aperture and an 800 mm receiver telescope on the ground station, the antenna gain is sufficiently high enough to perform QKD links. The LCT’s optical system is designed based on a commercially available reflective telescope in combination with custom-designed lenses. It integrates several fiber-optical and electrical components for wavelength combination, wavelength splitting, tracking, and uplink data reception into a compact design. The Alice payloads are connected to the LCT by optical fibres allowing a flexible placement of the modules within the satellite bus. Collimators couple the light into free-space beams which are co-aligned and interfaced to the transmit telescope. In addition to the optical system design, this paper provides further information about the integration of the optical into the mechanical system, and the performed functional and qualification tests.
Optical quantum communication utilizing satellite platforms has the potential to extend the reach of quantum key distribution (QKD) from terrestrial limits of ∼200 km to global scales. We have developed a thorough numerical simulation using realistic simulated orbits and incorporating the effects of pointing error, diffraction, atmosphere and telescope design, to obtain estimates of the loss and background noise which a satellite-based system would experience. Combining with quantum optics simulations of sources and detection, we determine the length of secure key for QKD, as well as entanglement visibility and achievable distances for fundamental experiments. We analyse the performance of a low Earth orbit satellite for downlink and uplink scenarios of the quantum optical signals. We argue that the advantages of locating the quantum source on the ground justify a greater scientific interest in an uplink as compared to a downlink. An uplink with a ground transmitter of at least 25 cm diameter and a 30 cm receiver telescope on the satellite could be used to successfully perform QKD multiple times per week with either an entangled photon source or with a weak coherent pulse source, as well as perform long-distance Bell tests and quantum teleportation. Our model helps to resolve important design considerations such as operating wavelength, type and specifications of sources and detectors, telescope designs, specific orbits and ground station locations, in view of anticipated overall system performance.
Satellite based Quantum Key Distribution (QKD) systems appear as a potential solution to cover long distance links that cannot be satisfied through fiber links. In this frame, different alternatives are being evaluated involving both geostationary and low earth orbit. Most existing or planned space based QKD missions focus on the build, analysis and performance optimization of the physical quantum channel. This represents only a tiny part of the complexities that imply the design and operation of a commercial oriented full operational end to end QKD system. Based on the state of the art development of the future Spanish GEO QKD system, this paper covers a systemic approach to space based QKD systems, focusing on some limiting factors that need to be taken into account in order to achieve continuous operation of a QKD system from space. Systems parameters such as the on-board memory or the different system delays and their impact on continuous operation are studied. Finally, the paper presents how these different interactions have been arbitrated in the current design of the Spanish GEO QKD system that shall be built, launched and operated before 2030 with the support of the Spanish government and under EC funding.
This paper presents the main architectural and technical elements of the Spanish initiative GARBO. This project, led by Thales Alenia Space Spain, with the support of Spanish authorities (CDTI), Operators (Hispasat) and other key industrial actors targets the definition, development, production and operation of a space based full end to end QKD system supported by a Prepare & Measure payload hosted and operated in a geostationary (GEO) telecommunications satellite. The target to operate from GEO orbit fixes strong constraints on the nature and operation of the system as will be shown in this paper. GARBO covers all R&D activities aimed at defining and demonstrating a complete quantum key transmission system (QKD) from geostationary orbit, to do so, as presented here, the GARBO team has designed the entire end-to-end system with special focus on the very stringent link budget for the quantum signal. Here, the optimal wavelength plan and has validated it through breadboard experiments. In the same way, both space and ground terminals are defined and presented. Finally, and being GARBO a project that targets full end to end operation, some overall system throughput analyses are drafted beyond the common Secure Key Rate figure directly linked to the QKD channel. Other higher level metrics and figures of merit are proposed to assess the adequacy of the proposed solution to the overall system needs and its potential integration with ground QKD networks such as the future EuroQCI.
We present a mission proposal to provide a space based quantum key distribution service. Using entangled photon pairs distributed to two ground stations with a simultaneous double downlink establishes a secret key directly at the user locations. The satellite and the payload do not generate or process any secret key information and the security design, certification and operation of the key management can be restricted to the user locations on ground. Positioning the satellite in GEO enables covering Europe and intercontinental connections, allows flexible service planning to cope with weather constraints and removes the need for coarse tracking telescopes in space and on ground while allowing long integration times. The large link distance and the combined losses of two down links require large optical terminals in space and on ground to obtain a usable secure key generation rate. We will present results from an ESA internal Phase 0 study concentrating on the payload design and performance.